The present invention relates, in general, to a process for depositing II-VI ternary semiconductor compounds as an epitaxial film onto a semiconductor substrate that is crystallographically compatible with said deposited film. In a more particular aspect, this invention concerns itself with the vapor phase deposition of a ternary mercury cadmium telluride compound as an epitaxial film onto a cadmium telluride substrate. The resulting film finds particular utility as an intrinsic photodetector for infrared applications in wavelengths of from about 1 to 30 micrometers.
As is well known by those skilled in semiconductor technology, II-VI semiconductor compounds are those which include elements from group II and group VI of the Periodic Table of Elements. The components of these compounds can be binary or ternary mixtures of any of those elements. Among one of the better known and more useful II-IV compounds is the ternary mixture of the elements mercury, cadmium and tellurium. Mercury-cadmium-telluride alloys, often referred to by their chemical designation of (Hg,Cd)Te or Hg.sub.1-x Cd.sub.x Te where x is an integer of greater than zero and less than one, have been found to be useful in a wide variety of semiconductor applications. They are especially useful and valuable as infrared detectors because in actuality they consist of a mixture of HgTe and CdTe. HgTe is characterized by a zero energy gap as opposed to a 1.5 e.v. energy gap possessed of CdTe. The final energy gap of the mixture can be predetermined by controlling the x value in order to render the mixture sensitive to a particular wavelength, thus providing the mixture with great versatility for use in numerous semiconductor applications. For example, a gap width of 0.1 e.v. provides the basis for using the (Hg,Cd)Te compound as an intrinsic infrared detector in the 8-14 micrometer range operable at 77.degree. K.
The tremendous interest in fabricating and utilizing infrared detection devices has spawned a considerable research effort in an attempt to grow high quality (Hg,Cd)Te compounds. A number of methods for accomplishing the synthesis of these ternary compounds are well known. Among the more important techniques is that involving the bulk growth of homogeneous crystals of the (Hg,Cd)Te alloy. In this technique, a liquid solution of predetermined composition is quenched to form a solid body of (Hg,Cd)Te. Next the solid body is annealed to remove dendrites. Finally, the (Hg,Cd)Te is annealed in the presence of mercury at a lower temperature than its first anneal to adjust its stoichiometry. The final anneal takes about 30 days. Unfortunately, the final product often contains foreign atoms or defects in the crystal lattice which provides undesirable trapping or recombination centers.
A number of other bulk growth techniques, such as zone melting, have also been resorted to; but, all require post growth processing. Also, the resulting crystal must be sliced, polished and etched prior to affixing it to a substrate by employing an epoxy adhesive. The additional step of using an epoxy adhesive poses even further problems in the creation of an undesirable thermal barrier between the (Hg,Cd)Te crystal film and the substrate.
In an attempt to overcome the problems associated with bulk grown crystals, those skilled in the art resorted to epitaxial film growth techniques. The epitaxial film is grown on a suitable substrate by vapor phase or liquid phase epitaxy. The resulting film is single crystal, smooth, continuous and crystallographically compatible with the substrate. It is characterized by uniformity in thickness and electrical properties. In addition, the utilization of epitaxial film techniques avoids the use of an epoxy bonding layer, as well as the other post growth procedures and their attendant problems.
The growth of epitaxial layers of (Hg,Cd)Te on cadmium telluride substrates, however, has been difficult to achieve by vapor phase epitaxial techniques because of the high pressure of the constituent elements. With the present invention, however, it has been found that high quality, detector grade, (Hg,Cd)Te epitaxial films can be grown effectively on a cadmium telluride (CdTe) substrate at moderate temperatures and pressures by the unique vapor phase technique of this invention. This technique, in general, involves the utilization of a double barrel reactor having three separate and distinct temperature zones for effecting the deposition of the (Hg,Cd)Te film.